An image forming apparatus of electrographic type is composed of an electrification device for uniformly electrifying an image carrier, an exposure device for exposing a surface of the image carrier to light so as to form an electrostatic latent image, a development device for making the electrostatic latent image into a visible image by developer, a transfer device for transferring the visible image onto a recording medium, a cleaning device for removing the developer that remains on the image carrier after the transfer, and a fixation device for fixing the visible image on the recording medium.
The image carrier, the electrification device, the exposure device, the development device, the transfer device, and the cleaning device, other than the fixation device, are referred to as process units. Though the process units are not required to be altogether integrated, it is typical for the image carrier, the electrification device, the development device, and the cleaning device to be integrated.
For the process units, long life, high reliability, high image quality, and low price have been required as well as reduction in size and speed-up of the devices, particularly, with increase of full color machines.
For the development device is dominant two-component development method using two-component developer that is typically composed of toner and carrier. The two-component development method makes it possible to perform development for a long period of time by continual replenishment of the toner, but has a problem in that the carrier deteriorates with lapse of time. In recent years, a trickle method has been employed for reducing deterioration of carrier by replenishment with not only toner but carrier on appropriate occasions and by discharge of extra carrier, so that long life and high reliability have been attained.
The devices other than the development device cannot technically be expected to have long lives because of wear in the image carrier, soiling on the electrification device, wear in cleaning blades of the cleaning device, and the like. The image carrier, the electrification device, and the cleaning device are different from the development device in life span, and therefore it is waste of resources to replace the devices in the lump. In some apparatuses, the image carrier, the electrification device, and the cleaning device are integrated into a drum cartridge, which is separated from the development device.
A clearance between the image carrier and a development roller of the development device, however, is extremely important for formation of uniform image densities. For the development device using two-component development, in particular, a difference in the clearance between the image carrier and the development roller along length (axes) thereof might lead to occurrence of image irregularity caused by rickety of the image carrier and the development roller. Accordingly, it is necessary to ensure a proper clearance between the image carrier and the development roller. In a structure as described above that has the drum cartridge and the development device separated from each other, it is difficult to ensure a clearance between the image carrier and the development roller.
In Patent Document 1 is proposed an imaging unit in which holes provided on both sides in the longitudinal direction of a housing of the drum cartridge and holes provided on both sides in the longitudinal direction of a housing of the development device are registered and engagement pins are fitted into the holes from outside. In Patent Document 1, rollers are provided on both sides in the longitudinal direction of a shaft of a development roller, the rollers are brought into contact with a surface of the image carrier at both end parts in the longitudinal direction by elastic members provided on both sides in the longitudinal direction, of the development roller, and a clearance is thereby established between the image carrier and the development roller.
Such a configuration as disclosed in Patent Document 1 has problems as follows. The drum cartridge and the housing of the development device which are formed by injection molding of resin material, undergo great change in dimensions due to change in temperature and have variations in the dimensions due to mass production. Accordingly, there is a fear that positions of the holes of the housing of the drum cartridge and a position of the image carrier might be deviated in the longitudinal direction and/or that positions of the holes of the housing of the development device and a position of the development roller might be deviated in the longitudinal direction. Besides, rickety may occur in fitting parts between the holes and the engagement pins engaged therewith. Then, in a situation that the drum cartridge and the development device are connected to each other, there is caused a variation in the longitudinal direction in positions of fulcrums on which the development device is pivoted relative to the drum cartridge. Therefore, distortion is produced in the unit and an angle of a development nip part is deviated in the longitudinal direction. Furthermore, the distortion in the unit causes one-side contact of the rollers with the surface of the image carrier and thus makes it impossible to maintain the clearance between the image carrier and the development roller with high accuracy. In addition, as the rollers are in contact with the surface of the image carrier at both the end parts with respect to the longitudinal direction, toner dispersed from the development device may adhere to the image carrier, soil the rollers, and thereby cause pitch irregularity.
In Patent Document 2, engagement between the development device and the drum cartridge is attained with use of shafts and grooves at four sites front and rear with respect to a longitudinal direction. The shafts and grooves at the four sites, however, do not necessarily coincide with each other and engagement of the shafts and grooves may cause distortion in the unit. It is therefore difficult to maintain a clearance between the development roller and the image carrier with high accuracy.    Patent Document 1: JP H10-142945 A    Patent Document 2: JP 2003-280490 A